1. Field of the Invention
This invention relates to a display device, a driving method and terminal device thereof and more particularly to the improvement on a problem occurring when switching between progressive driving and interlace driving is made and a problem relating to the power consumption.
2. Description of the Related Art
Recently, various types of mobile apparatus having display portions are developed. As one example of a mobile apparatus having a display portion, a notebook size personal computer (which is hereinafter referred to as a note PC) having a liquid crystal display device is provided. As one of factors which determine the value of the mobile apparatus having the display portion, the time during which the mobile apparatus can be driven by use of a battery is given. In order to increase the time during which the mobile apparatus can be battery-driven and used, it becomes necessary to suppress the power consumption of the whole mobile apparatus.
Generally, in the mobile apparatus having the display portion, the ratio of the power consumption of the display portion to the whole power consumption is high. Therefore, various methods for suppressing the power consumption of the display portion are developed. A case wherein the note PC having the liquid crystal display device is used as the mobile apparatus having the display portion is explained below.
As one example of the method for suppressing the power consumption of the note PC, a method for switching the driving methods of the liquid crystal display device according to the supply state of the power supply voltage is provided. For example, a method for making switching between progressive driving (or progressive mode) and interlace driving (or interlace mode) is provided. With this method, the liquid crystal display device is driven in a progressive (line-sequential) fashion in an environment in which electric power is always supplied from an electric outlet and driven in an interlace (thin-out) fashion in an environment in which it is supplied from a battery or the like (that is, in an environment in which supply of electric power is limited). In the case of interlace driving, the number of times by which the signal lines and scanning lines are driven in one vertical period are reduced to 1/n in comparison with the case of progressive driving and the interlace driving can be expected to extremely reduce the power consumption.
The technique for making switching between the interlace driving and progressive driving is disclosed in Patent Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2004-96223), for example. In this case, attention is paid to the fact that the precision of a converted signal is lowered in some cases when an interlace video signal is converted into a progressive video signal. In order to solve the above problem, a method is provided to detect the type of a sequence of an original interlace video signal and change the conversion processing state based on the detection result.
The interlace driving is a method for forming one image screen by superimposing (n+1) images in which n (n≧1) scanning lines are thinned out. In this method, the (n+1) images are images which complement one another. In the interlace driving, the number of times by which the signal lines and scanning lines are driven in one vertical period is reduced to 1/n. Therefore, the power consumption can be extremely reduced in comparison with that of the progressive driving. However, in the interlace driving, there occurs a problem that deterioration in the image quality, called line disturbance, occurs in a moving image portion.
Further, when attention is paid to a certain pixel, one driving operation is performed in one vertical period in the progressive driving, but the driving operation is performed only once in the (n+1) vertical periods in the interlace driving. If the interval in which the above driving operation is performed becomes long, the effective voltage applied to the pixel varies depending on the driving or on the property of the constituent material in some cases in the case of a liquid crystal display device. The phenomenon caused by the driving operation includes a leak phenomenon in which a pixel voltage is extracted via the parasitic capacitance formed between the signal line and the pixel electrode due to a video voltage signal supplied to the signal line. The phenomenon caused by the property of the constituent material includes natural discharging. Since the effective voltage applied to the pixel exerts an influence on the inclination of liquid crystal molecules, the luminance (transmittance) varies as a result. That is, even when the same gradation is displayed on the same liquid crystal display device, the luminance (brightness) becomes different in the case of progressive driving and in the case of interlace driving.
Further, in the interlace driving, the above (n+1) images are sequentially displayed. Therefore, when images having different colors and brightness with respect to the background are moved, deterioration in the display quality in which the boundary portion is observed as a jagged form (which is hereinafter referred to as “aliasing distortion”) or line disturbance may occur. This is because images (scanning lines) updated in each field become images thinned out for every n scanning lines. Therefore, when the progressive driving and interlace driving are switched in the liquid crystal display device, the luminance becomes different at the progressive driving time and at the interlace driving time even with the same gradation. Therefore, the luminance of the whole display screen is observed as if it varies.